When the race started shortly after 7 a.m. Sunday, the temperature was 60 degrees, with calm winds and 80 percent humidity. The temperature climbed to 64 at 8 a.m., 68 at 9 a.m., 72 at 10 a.m., 77 at 11 a.m. and 78 at noon, when the last of the participants were still on the course.

At least three runners were transported to hospitals, one for cardiac arrest and two for hyperthermia, Ryan said.

Even so, runners said conditions were better than in 2012, when oppressive heat and humidity forced organizers to halt the marathon before runners finished.

Before Sunday's start, runners took part in a tribute to Boston Marathon runners. Last month, that race was marred by two bomb blasts that killed three people and injured dozens.

Chris Troyanos, medical coordinator for the Boston Marathon, said the outpouring of support helps the city through a tough time.